Semiconductor memory device

ABSTRACT

A semiconductor memory device according to an embodiment comprises a memory cell array configured from a plurality of row lines and column lines that intersect one another, and from a plurality of memory cells disposed at each of intersections of the row lines and column lines and each including a variable resistance element. Where a number of the row lines is assumed to be N, a number of the column lines is assumed to be M, and a ratio of a cell current flowing in the one of the memory cells when a voltage that is half of the select voltage is applied to the one of the memory cells to a cell current flowing in the one of the memory cells when the select voltage is applied to the one of the memory cells is assumed to be k, a relationship M2&lt;2×N×k is satisfied.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. Ser. No. 16/875,259 filed May 15, 2020,which is a continuation of and claims the benefit of priority under 35U.S.C. § 120 from U.S. Ser. No. 15/475,276 filed Mar. 31, 2017, which isa continuation of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 14/860,805 filed Sep. 22, 2015 (now U.S. Pat. No.9,653,684 issued May 16, 2017), which is continuation of U.S. Ser. No.14/334,197 filed Jul. 17, 2014 (now U.S. Pat. No. 9,171,615 issued Oct.27, 2015), which is a continuation of U.S. Ser. No. 13/327,065 filedDec. 15, 2011 (now U.S. Pat. No. 8,848,418 issued Sep. 30, 2014), andclaims the benefit of priority under 35 U.S.C. § 119 from JapanesePatent Application No. 2011-6294 filed Jan. 14, 2011, the entirecontents of each of which are incorporated herein by reference.

FIELD

The embodiments relate to a semiconductor memory device.

BACKGROUND

In recent years, LSI elements configuring semiconductor memory devicesare becoming increasingly miniaturized as these semiconductor devicesbecome more highly integrated. Such miniaturization of LSI elementsrequires not only a simple narrowing of line width, but also improvementin dimensional accuracy and positional accuracy of circuit patterns.Proposed as a technology for overcoming these problems is ReRAM(Resistive RAM) which is configured by memory cells that include avariable resistance element and a selection element such as a diode. Amemory cell in this ReRAM does not require the use of a MOSFET andmoreover can be configured as across-point type. Hence, a high degree ofintegration exceeding conventional trends is expected of ReRAM.

However, in cross-point type architecture, a half-select bias system issometimes required. In this half-select bias system, a half-selectedcell current flows in addition to an ordinary selected cell current.Hence, when cell size undergoes reduction scaling, voltage drop withinthe memory cell array does not achieve a simple proportionalrelationship, and it is thus difficult to keep voltage drop constant.

Furthermore, when employing the half-select bias system, thehalf-selected cell current also gets mixed in with the selected cellcurrent during data read, thus making read of a selected cell difficult.When reduction scaling of cell size is performed, the proportion of thismixed-in half-selected cell current also increases, leading to problemsduring miniaturization of semiconductor memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an overall configuration of asemiconductor memory device according to a first embodiment.

FIG. 2 is a functional block diagram of the semiconductor memory deviceaccording to the same embodiment.

FIG. 3 is a circuit diagram of a memory cell array in the semiconductormemory device according to the same embodiment.

FIG. 4 is a characteristic diagram of a variable resistance element inthe semiconductor memory device according to the same embodiment.

FIG. 5A is a characteristic diagram of a selection element in thesemiconductor memory device according to the same embodiment.

FIG. 5B is a characteristic diagram of the selection element in thesemiconductor memory device according to the same embodiment.

FIG. 6 is a view explaining a circuit and bias states of the memory cellarray in the semiconductor memory device according to the sameembodiment.

FIG. 7 is a table showing effects due to scaling of various kinds ofparameters of the memory cell array in the semiconductor memory deviceaccording to the same embodiment.

FIG. 8 is a perspective view showing a structure of a memory cell unitin the semiconductor memory device according to the same embodiment.

FIG. 9 is a view explaining selection conditions of size of the memorycell array in the semiconductor memory device according to the sameembodiment.

FIG. 10 is a functional block diagram of a semiconductor memory deviceaccording to a second embodiment.

FIG. 11 is a perspective view showing a structure of a memory cell arrayblock in the semiconductor memory device according to the sameembodiment.

FIG. 12 is a view explaining a method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 13 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 14 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 15 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 16 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 17 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 18 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 19 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 20 is a view explaining the method of manufacturing the memory cellarray block in the semiconductor memory device according to the sameembodiment.

FIG. 21 is a view explaining a circuit and bias states of a memory cellarray in a semiconductor memory device according to a comparativeexample.

DETAILED DESCRIPTION

A semiconductor memory device according to an embodiment comprises: amemory cell array configured from a plurality of row lines and columnlines that intersect one another, and from a plurality of memory cellsdisposed at each of intersections of the row lines and column lines andeach including a variable resistance element; and a decoder for applyingto one of the memory cells a select voltage required in dataerase/write/read. Where a number of the row lines is assumed to be N, anumber of the column lines is assumed to be M, and a ratio of a cellcurrent flowing in the one of the memory cells when a voltage that ishalf of the select voltage is applied to the one of the memory cells toa cell current flowing in the one of the memory cells when the selectvoltage is applied to the one of the memory cells is assumed to be k, arelationship M²<2×N×k is satisfied.

A semiconductor memory device according to an embodiment is describedbelow with reference to the drawings.

First Embodiment

First, an overall configuration of a semiconductor memory deviceaccording to a first embodiment is described.

FIG. 1 is a perspective view showing the overall configuration of thesemiconductor memory device according to the first embodiment. A CMOScircuit 52 including a wiring layer is configured on an ordinary silicon(Si) substrate 51 (semiconductor substrate) by an ordinarily employedprocess, and a layer 53 including a plurality of memory cell units 54 isformed on the CMOS circuit 52. Each memory cell unit 54 shown in FIG. 1corresponds to a memory cell array 11 to be described later and haswiring formed by a 24 nm design rule. Moreover, a portion which includesa driver, a decoder, and a higher block and which is called a peripheralcircuit in an ordinary semiconductor memory device is included in theCMOS circuit 52.

Note that, excluding a connecting portion of the CMOS circuit 52 withthe memory cell unit 54, the CMOS circuit 52 is designed andmanufactured by a design rule of, for example, 90 nm which is morelenient than that of the memory cell unit 54. An electrical connectingportion with the CMOS circuit 52 (not illustrated) is provided in aperiphery of each memory cell unit 54. Blocks having these memory cellunit 54 and peripheral electrical connecting portion as a unit aredisposed in a matrix. Furthermore, a through hole (not illustrated) isformed in the layer 53 including the memory cell units 54. Theelectrical connecting portion of the memory cell unit 54 is connected tothe CMOS circuit 52 via this through hole. The memory cell unit 54 hasits operation controlled by the CMOS circuit 52. An input/output unit 55includes a terminal having an electrical joint with an input/output unitof the CMOS circuit 52. These terminals are also connected to theinput/output unit of the CMOS circuit 52 via the previously-mentionedthrough hole. Data, commands, addresses and so on required by the CMOScircuit 52 for controlling operation of the memory cell unit 54 areexchanged with external via the input/output unit 55. The input/outputunit 55 is formed at an end of the layer 53 including the memory cellunits 54.

The above configuration allows a portion corresponding to a protectivefilm of the CMOS circuit 52 to serve also as an insulating film formedin the memory cell unit 54. Moreover, in the present embodiment, thefact that the memory cell unit 54 and the CMOS circuit 52 join in astacking direction (Z direction) makes it possible to reduce operationtime without any associated increase in chip area, significantlyincrease the number of memory cells simultaneously accessible, and soon. Note that the input/output unit 55 is bonded to a lead frame in apackaging process, similarly to an input/output unit of an ordinarysemiconductor memory device.

Next, functional blocks of the semiconductor memory device according tothe present embodiment are described with reference to FIG. 2 .

This semiconductor memory device comprises the memory cell array 11including a plurality of row lines and column lines that intersect oneanother, and memory cells disposed at each of intersections of these rowlines and column lines. This memory cell array 11 corresponds to thememory cell unit 54 shown in FIG. 1 . In the explanation below, rowlines are called word lines, and column lines are called bit lines,after the example of an ordinary semiconductor memory device.

In addition, the semiconductor memory device comprises a row decoder 12for selecting a word line and a column decoder 13 for selecting a bitline during access (data erase/write/read). The column decoder 13includes a driver for controlling access operation.

Furthermore, the semiconductor memory device comprises a higher block 14serving as a control circuit for selecting access target memory cells inthe memory cell array 11. The higher block 14 provides a row address anda column address to, respectively, the row decoder 12 and the columndecoder 13. A power supply 15 generates certain combinations of voltagescorresponding to each of operations of data erase/write/read, andsupplies these combinations of voltages to the row decoder 12 and columndecoder 13.

The above functional blocks allow batch data erase/write/read of allmemory cells connected to an identical word line. The CMOS circuit 52shown in FIG. 1 is provided with peripheral circuits of the row decoder12, column decoder 13, and higher block 14, and so on shown in FIG. 2 .

Next, the memory cell array 11 in the semiconductor memory deviceaccording to the present embodiment is described with reference to FIG.3 .

The memory cell array 11 has a plurality of word lines WL and bit linesBL disposed intersecting one another, and has memory cells MC formed ateach of intersections of these word lines WL and bit lines BL, each ofthe memory cells MC including a variable resistance element VR. Aselection element S is connected in series to the variable resistanceelement VR of the memory cell MC, and the variable resistance element VRreceives supply of voltage from the word lines WL and bit lines BL viathis selection element S.

As a result of the above kind of structure of the memory cell array 11,the word lines WL and bit lines BL achieve a simple line-and-spacepattern, and, since, during formation of the memory cell array 11, theword lines WL and bit lines BL need only have a positional relationshipof intersecting one another, there is no need to consider misalignment.In other words, since alignment accuracy of the memory cells MC may begreatly relaxed, the semiconductor memory device can be easilymanufactured. Moreover, in the case of the above-described structure,one memory cell MC can be formed per 4F² region, hence a high degree ofintegration in the semiconductor memory device can be achieved.

The row decoder 12 is connected to each word line WL of the memory cellarray 11 and the column decoder 13 is connected to each bit line BL ofthe memory cell array 11. In addition, the row decoder 12 and columndecoder 13 are supplied with certain voltages corresponding to each ofoperations of data erase/write/read from the power supply 15.

Access to the memory cell MC is performed by, first, the row decoder 12and column decoder 13 selecting the word line WL and bit line BLconnected to an access target selected cell MC_(S) in the memory cellarray 11, based on a row address and column address outputted by thehigher block 14. In the case of FIG. 3 , the row decoder 12 and columndecoder 13 are arranged to select, respectively, the word line WL0 andbit line BL0.

Next, the row decoder 12 supplies a selected word line voltage VSW(selected row line voltage) to the selected word line WL (WL0 in FIG. 3), and supplies an unselected word line voltage VUW (unselected row linevoltage) to other unselected word lines WL. Meanwhile, the columndecoder 13 supplies a selected bit line voltage VSB (selected columnline voltage) to the selected bit line BL (BL0 in FIG. 3 ), and suppliesan unselected bit line voltage VUB (unselected column line voltage) toother unselected bit lines BL. Setting these selected word line voltageVSW, unselected word line voltage VUW, selected bit line voltage VSB,and unselected bit line voltage VUB to appropriate voltages to bedescribed later allows access to a certain memory cell MC in the memorycell array 11.

Next, characteristics of the memory cell MC are described.

First, characteristics of the variable resistance element VR in thememory cell MC are described with reference to FIG. 4 .

The variable resistance element VR is formed adopting a resistancevarying material typified by, for example, TiO₂, as its material. Thisresistance varying material is a material that undergoes transitionbetween at least two resistance values of a low-resistance state (LRS)and a high-resistance state (HRS).

The resistance varying material in the high-resistance state, whenapplied with a voltage of a certain level or more (a voltage of voltageVmset or more in a negative direction in FIG. 4 ), undergoes transitionto the low-resistance state as shown by arrow A1 in FIG. 4 . Such atransition of the resistance varying material from the high-resistancestate to the low-resistance state is called a “setting operation”. Datawrite in the present embodiment is realized by this setting operation.Note that in FIG. 4 , a current flowing in the resistance varyingmaterial during start of the setting operation is shown as Iset.

On the other hand, the resistance varying material in the low-resistancestate, when a current of a certain level or more (a current of currentIreset or more in FIG. 4 ) flows therein, undergoes transition to thehigh-resistance state as shown by arrow A2 in FIG. 4 . Such a transitionof the resistance varying material from the low-resistance state to thehigh-resistance state is called a “resetting operation”. Data erase inthe present embodiment is realized by this resetting operation. Notethat in FIG. 4 , a voltage applied to the resistance varying materialduring start of the resetting operation is shown as Vmreset.

In particular, the variable resistance element VR having the settingoperation and resetting operation performed by voltage applications ofdifferent polarities as shown in FIG. 4 is called a “bipolar operationelement”, and is employed in the memory cell MC in combination with theselection element S to be described later.

The variable resistance element VR may be configured by a thin filmhaving HfO_(x), ZnMn₂O₄, NiO, SrZrO₃, Pr_(0.7)Ca_(0.3)MnO₃, carbon, andso on, as its material, as an alternative to TiO₂.

Next, characteristics of the selection element S in the memory cell MCare described with reference to FIGS. 5A and 5B.

As previously mentioned, as a result of the variable resistance elementVR being a bipolar operation element, the selection element S must havecharacteristics of allowing a certain current of both positive andnegative polarities to flow as shown in FIGS. 5A and 5B. Therefore, adiode with large reverse direction leakage current, tunnel element, orthe like is employed as the selection element S.

The most important parameter as a characteristic of the selectionelement S is a half-selected cell current I_(H). The half-selected cellcurrent I_(H) herein refers to a current flowing in the memory cell MCwhen a voltage V_(S)/2 is applied to the memory cell MC, where a currentrequired in the setting operation/resetting operation of the variableresistance element VR is assumed to be a selected cell current I_(S),and a voltage applied to the memory cell MC to cause this selected cellcurrent I_(S) to flow is assumed to be V_(S) (selected voltage). Below,for convenience, a ratio of the selected cell current I_(S) to thehalf-selected cell current I_(H) is defined as k, and this k is assumedto be a characteristic parameter of the selection element S. Moreover, amemory cell MC applied with the voltage V_(S)/2 is called a“half-selected cell”.

Note that, strictly speaking, the memory cell MC is configured by thevariable resistance element VR and the selection element S connected inseries, hence, in order to be employed in an array operation to bedescribed later, must be corrected according to voltage distributions ofthese two elements.

Next, a size and bias state during data erase/write/read of the memorycell array 11 in the semiconductor memory device according to thepresent embodiment are described with reference to FIG. 6 .

The memory cell array 11 in the present embodiment has an array size ofM×N, and a relationship M²<2Nk satisfied between these M and N. Inparticular, in the present embodiment, as shown in FIG. 6 , when 2Nk issufficiently larger than M² (M²<<2Nk), advantages of the presentembodiment may be more greatly obtained. Therefore, described below isthe case where M²<<2Nk is satisfied, for example, the case where M=1Kand N=16K, and so on. Now, k is a previously mentioned characteristicparameter of the selection element S. Moreover, the unselected word linevoltage VUW and selected bit line voltage VSB both equal a certainvoltage V.

In the present embodiment, setting the array size and bias state of thememory cell array 11 as in FIG. 6 makes reduction scaling of the memorycell array 11 easier, and to help this point to be understood, the caseis described of a memory cell array shown in a comparative example shownin FIG. 21 . As an example, FIG. 21 shows the bias state in the casewhere the word line WL0 is assumed to be the selected word line, the bitline BL0 is assumed to be the selected bit line, and the memory cellMC_(S) connected to the intersection of these word line WL0 and bit lineBL0 is assumed to be the selected cell.

The memory cell array according to this comparative example has an arraysize M×N, where normally M and N are approximately equal. In addition,the unselected word line voltage VUW and unselected bit line voltage VUBare both V/2, and set to an intermediate voltage of the selected wordline voltage VSW and selected bit line voltage VSB.

In this case, a selected bit line current I_(B) shown by the solid linearrow in FIG. 21 flows in the selected bit line BL0. In addition, whenno account is taken of voltage drop, memory cells connected to theselected word line WL0 or selected bit line BL0 and distinct from theselected cell MC_(S) become half-selected cells MC_(H) applied with abias of V/2. A half-selected cell current I_(H) shown by the broken linearrows in FIG. 21 flows in these half-selected cells MC_(H).

When actually performing an array operation, it becomes important toapply the desired voltage V_(S) to the selected cell MC_(S),compensating for the voltage drop resulting from wiring in the memorycell array, and, in the case of FIG. 21 , voltage drop BL_IR and WL_IRof the selected bit line BL0 and selected word line WL have, as amaximum, values shown in expressions (1) and (2), respectively:

$\begin{matrix}{{BL\_ IR} = {{I_{S}\left( {2N\;\rho} \right)} + {\frac{1}{2}{{NI}_{H}\left( {2N\;\rho} \right)}}}} & (1) \\{{WL\_ IR} = {{I_{S}\left( {2M\;\rho} \right)} + {\frac{1}{2}{{MI}_{H}\left( {2M\;\rho} \right)}}}} & (2)\end{matrix}$

where ρ is a sheet resistance of the bit line BL and the word line WL.

The case is now considered of performing reduction scaling of a size ofa system with a scaling coefficient λ at constant current density. Thisscaling coefficient λ has a value that is smaller the more reduced isthe size of the system. In this case, the various parameters of thememory cell array change as shown in the table of FIG. 7 . Thus, uponconsideration of the scaling coefficient λ and setting I_(H)=I_(S)/k,expressions (3) and (4) are respectively obtained from expressions (1)and (2).

$\begin{matrix}{{BL\_ IR} = {{I_{S}\left( {2N\;\rho} \right)} + {\frac{1}{2}\frac{{NI}_{S}}{k}\left( {2N\;\rho} \right)\frac{1}{\lambda}}}} & (3) \\{{WL\_ IR} = {{I_{S}\left( {2M\;\rho} \right)} + {\frac{1}{2}\frac{{MI}_{H}}{k}\left( {2M\;\rho} \right)\frac{1}{\lambda}}}} & (4)\end{matrix}$

Looking at expressions (3) and (4), neither of the first terms on theright-hand side of each depends on λ, hence, even if reduction scalingis performed, a constant voltage drop can be maintained. On the otherhand, the second terms on the right-hand side are both proportional to1/λ, hence it is clear that if the system is reduced, the voltage dropincreases.

Similarly, if the proportion of the selected bit line current I_(B)flowing in the column decoder 13 taken up by the selected cell currentI_(S) when performing read of the cell current from the column decoder13 side is defined as READ signal rate, then its scaling dependency isgiven by the following expression (5).

$\begin{matrix}{{{READ}\mspace{14mu}{signal}\mspace{14mu}{ratio}} = {\frac{I_{S}}{I_{S} + {NI}_{H}} = \frac{1}{1 + \frac{N}{k\;\lambda}}}} & (5)\end{matrix}$

It is clear from expression (5) that the READ signal ratio also does notbecome constant during reduction scaling and if the system is scaleddown the proportion of the half-selected cell current I_(H) increaseswhereby detection of the selected cell current I_(S) becomes difficult.

In contrast, in the present embodiment shown in FIG. 6, a bias voltageof the unselected word line WL1 and so on is brought closer to a biasvoltage of the selected bit line BL0, hence the half-selected cellcurrent I_(H) does not get mixed in with the selected bit line currentI_(B). Therefore, the voltage drop of the selected bit line BL0 achievesa constant value given by expression (6), even if reduction scaling isperformed.

$\begin{matrix}{{BL\_ IR} = {I_{C}\left( {2\; N\;\rho} \right)}} & (6)\end{matrix}$

On the other hand, the voltage drop of the selected word line WL0 isgiven by the following expression (7) which is mathematically similar tothe case of the comparative example.

$\begin{matrix}{{WL\_ IR} = {{I_{S}\left( {2M\;\rho} \right)} + {\frac{1}{2}\frac{{MI}_{S}}{k}\left( {2M\;\rho} \right)\frac{1}{\lambda}}}} & (7)\end{matrix}$

However, as previously mentioned, when the array size is determined soas to satisfy M²<<2Nk, the second term on the right-hand side ofexpression (7) is sufficiently smaller than the value of expression (6)and in effect may be ignored. As a result, only the first term on theright-hand side of expression (7) is valid and the voltage drop WL_IR ofthe selected word line WL0 effectively attains a constant value even ifreduction scaling is performed.

Note that in the estimate of expression (6), it must be assumed that thevoltage drop of the unselected word line WL1 and so on is sufficientlysmall. However, the second term on the right-hand side of expression (7)and the voltage drop of the unselected word line WL1 and so on are givenby an identical mathematical expression, hence, when M²<<2Nk issatisfied, this assumption is also satisfied.

Furthermore, in the present embodiment, as previously mentioned, thereis no half-selected cell current I_(H) mixed into the selected bit linecurrent I_(B), hence the READ signal ratio is constantly 1 and is neverreduced even if reduction scaling is performed.

As is clear from the above, making the array size nonsymmetricalvertically and horizontally and bringing the bias voltage of theselected bit line BL closer to the bias voltage of the unselected wordline WL allows the voltage drop to in effect be maintained at a constantvalue during reduction scaling. Furthermore, the signal ratio of theread target selected cell current I_(S) can also be maintained at aconstant value.

Specifically, for example, when configuring the memory cell array 11 byone bit per cell memory cells MC and allocating 16M bits of storagecapacity to one memory cell array 11, if it is assumed that M=1K andN=16K, then M²=1M and 2N=32K. In this case, provided the parameter k ofthe selection element S is about 1000 or more, then in the firstgeneration M²/2Nk<1/32, and even after the third generation M²/2Nk<1/11,whereby a ratio having a size of one digit or more can be secured. Inaddition, in the case where the selection element S is a linear element,then k=2, but even in this case, if it is assumed that M=128 and N=256K,then in the first generation M²/2Nk=1/32, whereby a ratio of one digitor more can be secured similarly to the previous example.

Furthermore, in order to allocate a large storage capacity to anidentical chip area, a stacking structure may be adopted in which Llayers of M×N memory cell arrays 11 are stacked in a directionperpendicular to the substrate (Z direction). In this case, connectionto each of the bit lines BL must be performed on a one by one basis, butconnection to the word lines WL may for example be configured commonlyin even-numbered memory cell arrays 11 and odd-numbered memory cellarrays 11, respectively.

That concludes description of the case where M=1K and N=16K as anexample satisfying M²<<<2Nk. It is now described generally to whatdegree it is desirable for 2Nk to be larger than M².

In this description, “(right-hand side)-(left-hand side)” per unit bitline length is introduced as an evaluation function with respect to theinequality M²<<2Nk, and this evaluation function is described as f. Inaddition, (M, N, x) are adopted as independent variables in place of thegroup (M, N, k), (where x=M²/2Nk) In this case, f may be expressed as inexpression (8).

$\begin{matrix}{f = {\frac{M^{2}}{N}\left( {\frac{1}{x} - 1} \right)}} & (8)\end{matrix}$

FIG. 9 displays expression (8) as a graph. As is clear from FIG. 9 ,when (an absolute value of) a gradient of function f exceeds 1, fincreases rapidly (shaded region shown in FIG. 9 ), hence the evaluationfunction may be regarded as being sufficiently large. Conditions thatthe gradient of function f are greater than 1 are as in expression (9).

$\begin{matrix}{{\frac{\partial f}{\partial x}} = {{\frac{M^{2}}{N}\left( \frac{1}{x^{2}} \right)} > 1}} & (9)\end{matrix}$

From the above, conditions of desirable size of 2Nk with respect to M²may be expressed as in expression (10),

$\begin{matrix}{x < \frac{M}{\sqrt{N}}} & (10)\end{matrix}$

although it should be noted that f>0 in all cases hence x<1 must besatisfied, regardless of expression (10).

Note that the bias states shown in FIGS. 6 and 21 are simply examples,and only relative values of voltages between each of the electrodes issignificant. Therefore, for example, the combination (+V/2, 0, −V/2) mayalso be employed in place of the combination (V, +V/2, 0), bysubtracting V/2 overall as shown in brackets in FIGS. 6 and 21 .Although in this case, a circuit for generating negative voltagesbecomes necessary, there are advantages that the maximum voltage to besupplied by the circuit can be reduced, hence breakdown voltage of theCMOS circuit can be reduced and occupied area of the CMOS circuitportion can be reduced.

As described above, the present embodiment allows voltage drop in thewiring to be in effect maintained constant, and, furthermore, READsignal ratio in the wiring to be in effect maintained constant, evenwhen reduction scaling of cell size is performed. Therefore, the presentembodiment can provide a semiconductor memory device in which reductionscaling can be easily performed without the need to consider voltagedrop and READ signal ratio.

Second Embodiment

A semiconductor memory device according to a second embodiment differsfrom the semiconductor memory device according to the first embodimentmainly in a structure of a portion corresponding to the memory cell unit54 shown in FIG. 1 . Accordingly, the semiconductor memory deviceaccording to the present embodiment is described below focusing on thedifference with the semiconductor memory device according to the firstembodiment.

FIG. 10 is a view showing functional blocks of the semiconductor memorydevice according to the present embodiment.

The semiconductor memory device according to the present embodimentcomprises a memory cell array block 31, a column/layer decoder 33, and ahigher block 34 in place of, respectively, the memory cell array 11, thecolumn decoder 13, and the higher block 14, but otherwise comprisessimilar functional blocks to those of the semiconductor memory deviceaccording to the first embodiment.

The memory cell array block 31 is configured having a plurality ofmemory cell arrays stacked, each of the memory cell arrays including aplurality of word lines and bit lines that intersect one another andmemory cells provided at each of intersections of the word lines and bitlines.

The column/layer decoder 33 which includes a driver having a dataerase/write/read function is connected to each of the bit lines BL inthe memory cell array block 31. This column/layer decoder 33 selects aspecific memory cell array in the memory cell array block 31 based on acolumn/layer address outputted from the higher block 34, and suppliesthe selected bit line voltage VSB or the unselected bit line voltage VUBto bit lines of this memory cell array.

Next, each of the memory cell arrays in the memory cell array block 31is described.

FIG. 11 is a perspective view showing part of the memory cell arrayblock 31. The X direction, Y direction, and Z direction in FIG. 11 areidentical to, respectively, the X direction, Y direction, and Zdirection shown in FIG. 1 .

As shown in FIG. 11 , the memory cell array block 31 is configured by aplurality of memory cell arrays stacked with a certain pitch in the Ydirection.

Each of the memory cell arrays in the memory cell array block 31includes a plurality of bit lines BL arranged in a Z-X plane to extendin the X direction and having a certain pitch in the Z direction, aplurality of column-shaped word lines WL arranged in the Z-X plane toextend in the Z direction and having a certain pitch in the X direction,and memory cells MC provided at each of intersections of these bit linesBL and word lines WL. Now, the number M of bit lines BL and the number Nof word lines WL arranged in each of the memory cell arrays has therelationship of M²<2Nk, similarly to the memory cell array 11 accordingto the first embodiment. In particular, the fact that a greateradvantage can be obtained in the present embodiment when Nk issufficiently larger than M² (M²<<2Nk) is similar to in the firstembodiment. Note that the word lines WL and bit lines BL are shared bytwo memory cell arrays adjacent in the Y direction.

In addition, fellow word lines WL in odd-numbered memory cell arrays arecommonly connected by word line connecting lines WLCL (row lineconnecting lines) arranged in an X-Y plane to extend in the Y directionand having a certain pitch in the X direction. Similarly, fellow wordlines WL in even-numbered memory cell arrays are also commonly connectedby word line connecting lines WLCL.

Configuring a word line WL direction in the memory cell array as the Zdirection in this way allows an arrangement direction of the bit linesBL to be matched to a perpendicular direction to the silicon substrate51 which is most difficult for repeated formation, thereby allowingoptimization of the chip overall to be achieved.

Note that in the case of the second embodiment, the word lines WL arecommonly connected by the word line connecting line WLCL, henceconsideration must be given not only to voltage drop in the bit lines BLand word lines WL, but also to voltage drop in the word line connectingline WLCL. However, the word line connecting line WLCL may be disposedat an outer edge of the memory cell array block 31, hence sheetresistance can be reduced by a means such as increasing a filmthickness. As a result, effects due to the word line connecting lineWLCL can be reduced.

In the case of the semiconductor memory device according to the secondembodiment, if a ratio of a sheet resistance of the bit line BL to asheet resistance of the word line connecting line WLCL is defined to ber, and the relationship L²Mr<2Nk is satisfied in addition to therelationship M²<2Nk required in the first embodiment, voltage drop dueto the half-selected cell current I_(H) can be reduced. Particularly inthe case where the relationship M²<<2Nk is satisfied, and, furthermore,the relationship L²Mr<<2Nk is satisfied, voltage drop due to thehalf-selected cell current I_(H) may be ignored.

The second embodiment not only allows reduction scaling to be easilyperformed similarly to in the first embodiment but also allows an evenhigher degree of integration to be achieved than in the firstembodiment.

For example, if a semiconductor memory device in the first embodimentshown in FIG. 8 is assumed to be configured having M=1K, N=16K and L=8,and it is desired to realize a semiconductor memory device having thesame storage capacity, the same area, and the same voltage drop as thisby the semiconductor memory device according to the second embodimentshown in FIG. 11 , it is only required to configure that sheetresistance ratio r=0.25, M=8, N=16K and L=512.

This is because, in the case of the second embodiment shown in FIG. 11 ,the memory cells MC can be provided on both sides of each of the bitlines BL and each of the word lines WL. Moreover, in the case ofemploying a memory cell array block 31 thus configured, bringing theunselected word line voltage VUW closer to the selected bit line voltageVSB allows voltage drop due to the half-selected cell current I_(H) tobe ignored similarly to in the first embodiment.

Next, manufacturing processes of the memory cell array block 31 aredescribed with reference to FIGS. 12-20.

First, as shown in FIG. 12 , a CVD method is employed to stack,alternately, on one side of a silicon substrate 101 (semiconductorsubstrate), an interlayer insulating film 103 configured from SiO₂ and asilicon (Si) film 102 including a high concentration impurity. Uponcompletion of subsequent processing steps, this silicon film 102 becomesthe bit lines BL shown in FIG. 11 . As a result, the same number oflayers of the silicon film 102 are stacked as there are bit lines BLformed in the Z direction perpendicular to the silicon substrate 101 (4layers in FIG. 11 ).

Then, as shown in FIG. 13 , an etching mask 106 is stacked via aninsulating film 104 and an insulating film 105. A resist pattern informed on the etching mask 106 using a photo etching process. Theetching mask 106 undergoes patterning by reactive ion etching with thisresist pattern acting as a mask. The etching mask 106 is formedextending in the X direction and arranged in a plurality of lines in theY direction.

Subsequently, as shown in FIG. 14 , a mask material is deposited on theinsulating film 105 and the etching mask 106, then etching is performed.As a result of this etching, aside wall mask 107 extending along a Ydirection side wall of the etching mask 106 is formed.

Then, as shown in FIG. 15 , the interlayer insulating film 103 and thesilicon film 102 are etched by reactive ion etching with the etchingmask 106 and the side wall mask 107 acting as a mask. This etching isperformed until the silicon substrate 101 is reached and a surface ofthe silicon substrate 101 is exposed.

Subsequently, as shown in FIG. 16 , a resistance varying material 108 isformed on a side surface of the silicon film 102 exposed by the etching.Then, a silicon (Si) film 109 including a high concentration impurity isdeposited to fill in between the resistance varying material 108. Uponcompletion of subsequent processing steps, this silicon film 109 becomesthe word lines WL shown in FIG. 11 . The silicon film 109 is connectedto diffusion layer wiring (not illustrated) provided beforehand on thesilicon substrate 101.

Subsequently, as shown in FIG. 17 , an etching mask 110 for use in alater etching process is deposited on the resistance varying material108 and the silicon film 109. Then, the etching mask 110, the etchingmask 106, and the side wall mask 107 are planarized by CMP (ChemicalMechanical Polishing).

Next, as shown in FIG. 18 , the etching mask 106 only is removed.Subsequently, the interlayer insulating film 103 and the silicon film102 are etched by reactive ion etching with the etching mask 110 and theside wall mask 107 acting as a mask. This etching is performed so as notto reach the silicon substrate 101, that is, the etching allows theinterlayer insulating layer 103 to remain.

Subsequently, as shown in FIG. 19 , a resistance varying material 111 isformed on a side surface of the silicon film 102 exposed by the etching.Then, a silicon (Si) film 112 including a high concentration impurity isdeposited to fill in between the resistance varying material 111. Uponcompletion of subsequent processing steps, this silicon film 112 becomesthe word lines WL shown in FIG. 11 . The silicon film 112 is insulatedand isolated from the silicon substrate 101 on which the diffusion layerwiring is formed, by a lowermost layer of the interlayer insulating film103. Then, CMP is used to planarize the silicon film 112 and remove theetching mask 107.

Finally, as shown in FIG. 20 , a metal film and an etching mask aredeposited on all surfaces, then a photo etching process is employed toform a resist pattern on the etching mask. The etching mask undergoespatterning by reactive ion etching with this resist pattern acting as amask. The etching mask is formed extending in the Y direction andarranged in a plurality of lines in the X direction. The metal film andthe silicon film are etched by reactive ion etching with this etchingmask acting as a mask. This etching causes the silicon film to beseparated into a plurality of word lines WL aligned in the X direction.Moreover, etched metal wiring 113 becomes the word line connecting lineWLCL shown in FIG. 11 . Note that the metal wiring 113 is insulated andisolated from the silicon film 109 by the etching mask 110.

The above processes enable the semiconductor memory device shown in FIG.11 to be manufactured. A photo etching process is performed twice in theabove-mentioned method of manufacturing, thereby allowing any rise inlithography processing costs to be suppressed. In addition, the siliconfilm 109 (word line WL) and the silicon film 112 (word line WL) formedalternately between the silicon film 102 (bit line BL) are formed in anopening etched with the etching masks 106 and 110 formed alternatelybetween the side wall mask 107 acting as a mask. If the photo etchingprocess is performed a plurality of times when manufacturing the wordlines WL, it is easy for misalignment to occur, leading to a risk ofvariation in performance of the manufactured word lines WL, memory cellsMC, and so on. However, the above-mentioned method of manufacturing hasalignment of word lines WL performed without removing the side wall mask107, hence enables misalignment, variation in line width, and so on, tobe suppressed.

As described above, the present embodiment makes it possible to providea semiconductor memory device which not only enables similar advantagesto those of the first embodiment to be obtained but also allows an evenhigher degree of integration to be achieved than in the firstembodiment.

[Other]

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A semiconductor memory device, comprising: asubstrate; a first wiring extending in a first direction parallel to asurface of the substrate; a second wiring being apart from the firstwiring in a second direction crossing the first direction, the secondwiring extending in the first direction; a third wiring being providedbetween the first wiring and the second wiring, the third wiringextending in a third direction crossing in the first and the seconddirections and the third wiring including a first end portion of two endportions, the first end portion being located further from the surfaceof the substrate than the other end portion; a fourth wiring being apartfrom the third wiring in the second direction, the fourth wiringextending in the third direction and the fourth wiring including asecond end portion of two end portions, the second end portion beinglocated closer from the surface of the substrate than the other endportion; a first memory cell being provided between the first wiring andthe third wiring; a second memory cell being provided between the secondwiring and the third wiring; a third memory cell being provided betweenthe second wiring and the fourth wiring; a fifth wiring being connectedto the first end portion and extending in the second direction; a sixthwiring being connected to the second end portion and extending in thesecond direction; and a circuit including a first decoder and a seconddecoder, the first decoder being connected to the fifth wiring and thesixth wiring, the second decoder being connected to the first wiring,wherein the first decoder applies a first voltage to the fifth wiringand a second voltage to the sixth wiring, and the second decoder appliesa third voltage to the first wiring.
 2. The device according to claim 1,wherein a difference between the first voltage and the third voltagecorresponds to a selected cell voltage and generates erase/write/readcurrent.
 3. The device according to claim 1, further comprising: afourth memory cell being provided between the third wiring and a seventhwiring that are provided above the first wiring, wherein the seconddecoder further applies a fourth voltage to the seventh wiring.
 4. Thedevice according to claim 1, wherein the second decoder further appliesa fourth voltage to the second wiring.
 5. The device according to claim1, wherein the surface of the substrate is a surface on which a circuitincluding a wiring layer is configured.
 6. The device according to claim1, wherein the surface of the substrate is a surface extending in thefirst direction and the second direction.
 7. The device according toclaim 1, further comprising: an eighth wiring being adjacent to thesecond wiring in the second direction and extending in the firstdirection; a ninth wiring being adjacent to the eighth wiring in thesecond direction and extending in the first direction; a tenth wiringbeing provided between the eighth wiring and the ninth wiring, the tenthwiring extending in the third direction, the tenth wiring including athird end portion of two end portions, the third end portion beinglocated further from the surface of the substrate than the other endportion; and an eleventh wiring being adjacent to the tenth wiring inthe second direction, the eleventh wiring including a fourth end portionof two end portions, the fourth end portion being located closer fromthe surface of the substrate than the other end portion, wherein thefifth wiring is connected to the third end portion, and the sixth wiringis connected to the fourth end portion.
 8. The device according to claim7, further comprising: a twelfth wiring being provided between the firstwiring and the second wiring, the twelfth wiring extending in the thirddirection, the twelfth wiring including a fifth end portion of two endportions, the fifth end portion being located further from the surfaceof the substrate than the other end portion; a thirteenth wiring beingprovided between the eighth wiring and the ninth wiring, the thirteenthwiring extending in the third direction, the thirteenth wiring includinga sixth end portion of two end portions, the sixth end portion beinglocated further from the surface of the substrate than the other endportion; and a fourteenth wiring being adjacent to the fifth wiring inthe first direction, the fourteenth wiring being connected to the fifthend portion and to the sixth end portion, extending in the seconddirection.
 9. The device according to claim 8, wherein the twelfthwiring is adjacent to the third wiring in the first direction, and thethirteenth wiring is adjacent to the tenth wiring in the firstdirection.
 10. The device according to claim 8, wherein the thirdwiring, the fourth wiring and the tenth wiring are aligned in seconddirection, and the third wiring and the tenth wiring are not connectedto the sixth wiring.
 11. A semiconductor memory device, comprising: afirst wiring extending in a first direction; a second wiring being apartfrom the first wiring in a second direction crossing the firstdirection, the second wiring extending in the first direction; a thirdwiring being provided between the first wiring and the second wiring,the third wiring extending in a third direction crossing in the firstand the second directions and the third wiring including a first endportion of two end portions; a fourth wiring being apart from the thirdwiring in the second direction, the fourth wiring extending in the thirddirection and the fourth wiring including a second end portion of twoend portions; a first memory cell being provided between the firstwiring and the third wiring; a second memory cell being provided betweenthe second wiring and the third wiring; a third memory cell beingprovided between the second wiring and the fourth wiring; a fifth wiringbeing connected to the first end portion and extending in the seconddirection; a sixth wiring being connected to the second end portion andextending in the second direction; and a circuit including a firstdecoder and a second decoder, the first decoder being connected to thefifth wiring and the sixth wiring, the second decoder being connected tothe first wiring, wherein the first decoder applies a first voltage tothe fifth wiring and a second voltage to the sixth wiring, and thesecond decoder applies a third voltage to the first wiring, the firstend portion being located further from the circuit than the other endportion, and the second end portion being located closer from thecircuit than the other end portion.
 12. The device according to claim11, wherein a difference between the first voltage and the third voltagecorresponds to a selected cell voltage and generates erase/write/readcurrent.
 13. The device according to claim 11, further comprising: afourth memory cell being provided between the third wiring and a seventhwiring that are provided above the first wiring, wherein the seconddecoder further applies a fourth voltage to the seventh wiring.
 14. Thedevice according to claim 11, wherein the second decoder further appliesa fourth voltage to the second wiring.
 15. The device according to claim11, wherein the circuit is provided on a substrate above a surface ofthe substrate.